Industry has long relied upon a concept for preventive maintenance which is based upon the detection, analysis, and correction of vibration in monitored machinery. Initially, periodic vibration measurements were made with hand-held devices at various locations upon such machinery, whereupon changes in vibration characteristics were noted as representing a possible condition of machine deterioration. More elaborate vibrational analyses, for example involving vibration frequency spectrum approaches, then could be employed in diagnostic endeavors. As this general approach to preventive maintenance grew, many industrial entities implemented vibratory monitoring system in which a significant number of vibration transducers or pick-ups were permanently installed at various locations upon the machinery and their vibration related outputs monitored on a somewhat continuous basis with respect to predetermined warning and trip alarm levels. Such long term monitoring systems are described, for example, in U.S. Pat. Re. No. 31,750 and U.S. Pat. No. 4,399,513.
The pick-ups or transducers employed with the vibration responsive systems generally are structured to evaluate selected characteristics of vibration which typically are characterized as displacement, velocity and acceleration. Vibration velocity data long has been selected by industry as a direct measure of vibration severity. Displacement measurements have found considerable application, for example, in evaluating stress at low frequencies in machinery. Vibration acceleration measurements are employed to evaluate vibrating forces applied to machines under investigation. Accelerometer type transducers used for the latter data gathering function exhibit desirably flat response characteristics over extended frequency ranges and are somewhat conveniently, compactly structured. In general, the devices employ a known mass with a piezoelectric crystal to develop charge values reflecting characteristics of force and mass from which acceleration signals readily are develped. An important application for accelerometers resides in their specific employment for detecting the very high frequencies associated with impending bearing failure. Because of their advantageous sensitivity to high frequency vibrations, the technique for mounting accelerometers has been considered somewhat more critical than those approaches employed with other transducer forms. It is essential not only that mechanical decoupling with a vibratory surface be avoided, but also that the coupling itself not exhibit elasticity. For example, the use of elongated mounting bolts is avoided in view of their low spring constants and contribution to a low resonant frequency for the mounting. In a substantial number of mounting applications, a permanent arrangement is involved wherein thin cement layers are employed with an intermediate layer of insulative material lying between the accelerometer confronting face and the monitored vibratory surface.
Over the recent past, accelerometer sensors have been combined with permanet monitoring systems to achieve an automatic tool action monitoring function. With these systems, the accelerometer pick-ups are mounted at strategic positions on a machine tool and serve to respond to distinct and repeatable patterns of vibrational phenomena, changes to which provide early warning of tool wear or breakage. These early warnings so provided by the system enable operators to gain maximum tool life as well as part quality. However, for the systems to optimally perform, it usually is necessary to mount the accelerometer sensors on spindles or tool holders or the like which function to manipulate or drive tools. Such mounting requires a very high level of integrity to maintain required frequency response characteristics and this integrity must be maintained while the sensors are attached to moving machine component within a factory environment which is quite severe. For example, the environment includes cutting fluids, chips, dirt, oil, humidity, and elevated temperatures. Thus, the sensors can eventually fail due to coolant or other contaminants or mechanical damage. Additionally, because the spindle mechanisms and the like are removed and disassembled for maintenance on a relatively frequent basis, accommodation must be made for the accelerometer sensors during such procedures. Conventional mounting approaches lead to frustration and, often, loss of the sensing function in consequence of operator frustration due to the difficulties attendant with the removal and re-installation of sensors, connected signal carrying wiring, and the like. Thus, to maintain monitoring effectiveness over reasonable system lifespans, it is necessary that some form of mounting for the accelerometer sensors be achieved which is readily mounted and removed to facilitate machine maintenance operations while maintaining the frequency response performance integrity, a condition heretofore requiring rigid and permanent mounting practices.